1. Field of the Invention
The present invention relates generally to PID (Proportional-Integral-Derivative) controllers and more particularly to PID controllers having inner and outer loops of which the various gains influence input/output and perturbation sensitivity properties with a higher degree of decoupling.
2. Brief Description of the Prior Art
The PID controller remains, more than five decades after its adoption, the most popular and the most widely used theoretical as well as industrial controller. Its general properties in regard to effectiveness, simplicity and conditions of applicability are well recognized. Following extensive industrial experience a number of particularly attractive configurations have been proposed and various tuning procedures have been developed.
However, tuning the various gains of a PID controller still involves excessive costs, long start-up times and less than optimal operating conditions. This problem continues to inspire research aimed at refining and expanding the available PID state of knowledge. In particular, efforts are being devoted to improving the current tuning procedures.
Conventional PID controllers comprise a number of gains to control the various characteristics of the feedback. These characteristics are basically the transient response, the stationary gain (often referred to as the DC gain) and the sensitivity to perturbations. In a basic PID controller, each of these characteristics is usually viewed as being controlled by a separate single gain. An additional gain is usually introduced as a fine adjustment of both input/output and sensitivity response.
Present PID configurations show undesirable tuning characteristics. In particular, the various adjustable gains of a conventional PID configuration are somewhat linked, i.e. the adjustment of one gain modifies the characteristics controlled by the other gains. Therefore, each time a gain is tuned, re-adjustment of the other gains is usually required to maintain the desired characteristics of the PID controller.
Tuning methods have been elaborated to minimize the number of adjustments required but several iterations are usually needed before a satisfactory setting is reached. Once the tuning has been completed, doubts often persist as to whether the obtained setting is truly optimal or whether the operator has simply given up looking for a better one.